The knee comprises three inter-dependent joints in three separate compartments, all surrounded by a fibrous capsule covered by the skin. The medial tibio-femoral joint involves contact between the thigh bone (the femur) and the leg bone (the tibia) on the inside of the lower limb. The lateral tibio-femoral joint involves contact between the femur and the tibia on the outside of the lower limb. The patello-femoral joint involves contact between the femur and the knee cap (the patella) on the front of the lower limb.
The front of the lower (distal) end of the femur comprises an anticlastic flanged groove, convex in the sagittal plane, transversely concave, providing a track for the patella. The back of the distal femur divides into two separate near-spherical convex condyles making contact with the tibia. The upper surface of the tibia is like a plateau which is slightly dished on the medial side for contact with the medial femoral condyle forming the medial tibio-femoral joint and slightly convex on the lateral side for contact with the lateral femoral condyle forming the lateral tibio-femoral joint with a protrusion (the tibial eminence) running from front to back between the joints.
The articulating surfaces in each joint are covered with thin layers of a tough protective layer called cartilage, and are lubricated by synovial fluid secreted from a membrane on the inner surface of the fibrous capsule surrounding the knee. The surfaces of the tibio-femoral joints are further separated by the menisci, semi-circular semi-lunar collagen bundles oriented circumferentially. Each bundle is securely attached at each end to the tibia. The menisci form closely-fitting mobile sockets for the femoral condyles bringing the dissimilar surfaces of the femur and tibia into closer conformity.
The bones are held together actively by muscles with their tendons which span the joints and passively by ligaments and the joint capsule. The ligaments comprise bundles of collagen fibres running mainly longitudinally. The collateral ligaments arise on the external surfaces of the medial and lateral condyles. The medial collateral ligament inserts into the external medial surface of the proximal tibia. The lateral collateral ligament inserts into the proximal surface of the fibula. The medial collateral ligament is a much larger and stiffer structure than the lateral collateral ligament. The cruciate ligaments arise from the internal surfaces of the femoral condyles and insert into the tibial eminence.
The ligaments and the bones together form a mechanism which controls a complex pattern of movement of the bones on each other. In the unloaded state, flexion of the knee to 130° about a transverse axis is accompanied by approximately 25°rotation about the axis of the tibia (axial rotation) and approximately 5° about an anteroposterior axis (abduction-adduction). These movements are accommodated by mainly anteroposterior translations of the tibio-femoral contact areas so that the bones roll as well as slide on each other and the patella slides over the anterior femur.
Under load, the ligaments stretch and the articular surfaces indent, significantly modifying the relationship between flexion, axial rotation and abduction-adduction and between flexion and contact area translations. Movements at the knee are therefore load and activity dependent.
Damage to the articular surfaces or to the ligaments changes the patterns of movement of the bones on each other and the response of the joint to load. Osteoarthritis follows from failure of the cartilage in one or other of the three joints, leading to bone-on-bone contact and the onset of pain. Frequently, osteoarthritis first manifests itself in the medial compartment, while the ligaments remain intact. The disease can remain confined to the medial compartment until the anterior cruciate ligament fails and the disease then spreads to the other two compartments. No drug treatment has been found which reverses these processes.
Total knee replacement is the most common surgical treatment for osteoarthritis, involving replacement of the articular surfaces of all three compartments and sacrifice of some of the ligaments. Partial knee replacement involves replacement of the articular surfaces in only one compartment, leaving intact the surfaces of the other two compartments and all of the ligaments. Partial knee replacement can act prophylactically, reducing the rate of development of the disease in the other compartments. Partial knee replacement is surgically more demanding and is not always used when it is indicated.
To implant the prosthetic components of a knee replacement, sufficient sections of bone have to be removed from the surfaces of the tibia and the femur. The component parts of the prosthesis are then fitted accurately replacing the material removed by the surgeon. Care has to be taken not to overstuff the joint, leading to pain and failure of the components and, in the case of a partial knee replacement, degeneration of the preserved compartments.
Mobile bearing arthroplasty uses metal components fixed to the tibia and the femur with an intervening plastic bearing, an analogue of the natural meniscus, interposed between. The metal components are fixed to the bones so as to leave a constant gap between them when the knee is flexed and extended. The surgeon then selects the most appropriate thickness of bearing to fill the gap. The bearing is pushed between the metal components against the resistance of stretching ligaments. The bearing snaps into position once its thick section has passed through the thinnest section of the gap between the fixed components. A complication of mobile bearing arthroplasty is dislocation of the bearing. Dislocation rarely occurs after medial partial knee replacement but is the main complication of lateral arthroplasty. The surgeon naturally seeks to use the thickest possible bearing to avoid dislocation. However, if too thick a bearing is selected, there is a risk of overstuffing the joint, which can lead to pain and failure of the components and, in the case of a partial knee replacement, degeneration of the preserved compartments.
The difference between the maximum and minimum thickness of a mobile bearing is a measure of the entrapment of the bearing when it is held in place between the fixed metal components by the ligaments and a measure of the ligament stretch needed to implant or dislocate the bearing in use. A spherically convex tibial component (the domed tibial), similar in shape to the natural lateral tibial plateau, and a bi-concave meniscal bearing were developed to increase entrapment and to reduce the incidence of bearing dislocation after lateral arthroplasty. Nonetheless, lateral bearing dislocation continues to be a problem.
Complete dislocation can occur along the anteroposterior axis of the knee replacement. This can happen either in the anterior direction, as the reverse of the process of implantation, or in the posterior direction. If the patient distracts the joint i.e. by applying appropriate varus or valgus load to the limb, the bearing may be free to move through the enlarged minimum gap and escapes from the embrace of the femoral and tibial components which will then come into contact.
It is more likely to occur if the bearing is tipped at an angle so that it slides out of the gap along the femoral component.
The amount of distraction of the gap between the femoral component and the domed tibial component used in lateral arthroplasty required to cause dislocation can be expressed as follows:
For complete dislocation along tibia, the distraction required is:Dt=√{square root over (x12+(t1+t+R2)2)}−(R2+t)
For complete dislocation along the femur, the distraction required is:Df=√{square root over (x22+(t2+t+R1)2)}−(R1+t)whereR1 is the radius of the femoral componentR2 is the radius of the tibial componentx1 is the length of the posterior portion of the meniscal bearing at upper interfacex2 is the length of the posterior portion of the meniscal bearing at lower interfacet is the minimum thickness of the meniscal bearingt1=R1=√{square root over (R12−x12)} is the depth of the upper concavityt2=R2=√{square root over (R22−x22)} is the depth of the lower concavity
It is also possible for complete dislocation to occur in the lateral direction (towards the outside of the knee). Partial dislocation can occur in the medial direction (towards the inside of the knee) when the bearing moves medially and above the free end of the medial wall of the tibial component before becoming trapped between the fixed components. Partial medial dislocation and complete lateral dislocation continue to be a problem in lateral arthroplasty. Dislocation can also occur in a medial arthroplasty.
We have found it useful herein to define the directions ‘inner’ and ‘outer’ when applied to a knee. As used herein, the term ‘inner’ means central to the knee. Thus the side of a femoral or tibial component which is intended for location closest to the centre of the knee is the ‘inner’ side. In a lateral arthroscopy, the inner side is the medial side. In a medial arthroscopy, the inner side is the lateral side. Conversely, the term ‘outer’ is used herein to mean towards the outside of the knee (along a medial-lateral axis). Thus the side of a femoral or tibial component which is intended for location away from the centre of the knee is the ‘outer’ side. In a lateral arthroscopy, the outer side is the lateral side. In a medial arthroscopy, the outer side is the medial side.